The invention relates to the activation of chromium-containing supported catalysts. In another of its aspects, the invention relates to the production of polymers of alpha-olefins in the presence of chromium-containing supported catalysts which have been activated.
Chromium-containing supported catalysts useful for the polymerization of alpha-olefins have been disclosed in U.S. Pat. No. 2 825 721 issued to J. P. Hogan and R. L. Banks. As disclosed in this patent, catalysts containing chromium oxide associated with a support like e.g. silica, are usable for the polymerization of alpha-olefins. These catalysts can be activated by calcination in a non reducing atmosphere for a time sufficient to leave at least a portion of the chromium present in the hexavalent state.
Another method of activation of this type of catalyst has been disclosed in U.S. Pat. No. 3 362 946 issued to J. P. Hogan. As disclosed in this patent, it is possible to increase the activity of chromium-containing supported catalysts, after the catalyst has been activated, by treatment thereof with carbon monoxide at temperatures comprised between about 370.degree. and 540.degree. C.
However, activation treatments of chromium-containing supported catalysts implying the use of carbon monoxide are of course cumbersome.
Further, a decrease of the bulk densities and of the melt indexes of the polyolefins manufactured in the presence of catalysts activated in this way is frequently observed. Finally, the catalysts activated in this way are extremely sensitive to reoxidation.